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Old 4th August 2009, 09:37 PM   #21
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Quote:
Originally posted by darkfenriz
seriously current conveyor is a similar concept to operational amplifier, but it has two low impedance inputs and a high impedance output, nomen omen.
It has a intrisic speed, because there is no voltage gain inside.

Your patterns may work, but every transimpedance stage made around a resistor in negative feedback loop may have problems with stability.
It's because from 'voltage' point of view, input impedance is infinite, and resistance in feedbackloop is given, so the 'voltage' gain is below 1, which is something most opamps hate.
That's why I am going to use video opamps.

Are current conveyors available as ICs?
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Old 4th August 2009, 09:46 PM   #22
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This one looks better, for 20 mA input currents:

Click the image to open in full size.
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Old 4th August 2009, 10:01 PM   #23
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Actually i am not concious of any current convoyour IC.
edit: maybe you may use a current feedback amplifier with non-inverting input grouded
As for opamp stability, no opamp likes voltage gain below one, especially fast video opamps. A capacitor in feedback loop is often mandatory. I suggest you studdied circuits designed for fast photodiode amplifiers, they serve the same purpose which is converting current to voltage and there is much research there, for example on how to tailor feedback capacitor for macimum gain*bandwidth.
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Old 4th August 2009, 10:08 PM   #24
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Wavebourn,
just like in audio, overall negative feedback is really a nuisance here.
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Old 4th August 2009, 10:58 PM   #25
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Quote:
Originally posted by Lumba Ogir
Wavebourn,
just like in audio, overall negative feedback is really a nuisance here.
Which overall negative feedback?




The first one is a current to voltage converter, with close to zero input resistance to prevent cross-talk between signals I measure.

The second one, is a maximum-possible-gain amp with logarythmic overdrive function; the 3'd one has a positive feedback, a flip-flop with well defined hysteresis to shape output and prevent oscillations.

Do you know what are you talking about, Lumba?
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Old 5th August 2009, 06:06 AM   #26
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'Which overall negative feedback?'

That in the 1st opamp e.g., the I/V converter. The FB ratio is defined by Zol/R1
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Old 5th August 2009, 07:35 AM   #27
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Quote:
Originally posted by PMA
'Which overall negative feedback?'

That in the 1st opamp e.g., the I/V converter. The FB ratio is defined by Zol/R1
Oh eah, Lumba spotted it right away! No good for audio!


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Old 5th August 2009, 07:38 AM   #28
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Quote:
Originally posted by darkfenriz
maybe you may use a current feedback amplifier with non-inverting input grouded
Like is drawn on my pictures?

Thanks!
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Old 5th August 2009, 07:58 AM   #29
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What you specified you want in the first post and what you are drawing don't seem to relate.

What exactly is the function required?
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Old 5th August 2009, 08:12 AM   #30
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I need to compare 2 currents when they flow in different directions, if to say strictly. Both may swing between -20 mA to +20 mA, up to 1 MHz frequency.

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